Various kinds of illuminators are known in the art for illuminating display panels, such as transmissive and reflective liquid crystal displays (“LCDs”) as used in cellular phones, PDAs, electronic equipment, desktop and portable computers, as well as keyboards, and instrument panels to name but a few applications. The most recent type of these illuminators is of an “edge coupled” design, which ensures luminance uniformity over a display area with a relatively small device thickness. While luminance uniformity is important, device thickness is of crucial importance in flat panel displays (“FPDs”) and the other aforementioned applications.
A front and side view of a prior art flat illumination devices (“FIDs”) 10 as shown in FIG. 1A has one or more point-like or extended light sources 12a, 12b, 12c, 12d, 12e, 12f coupled to one or more edge faces 14a and 14b of a light guiding plate (“LGP”) 16 with generally non-uniformly distributed light extraction means. Point-like light sources most typically include light emitting diodes (“LEDs”) while typical extended light sources include cold cathode fluorescent lamps (“CCFLs”). Such prior art FIDs suffer from a number of well known drawbacks, namely poor brightness and color uniformity, especially when operating with LEDs, and low output efficiency. Because of these deficiencies, the number of lamps has to be increased, leading in turn to higher power consumption and device cost. The list of drawbacks would not be complete without mentioning the thickness of the device. For all the light emanating from the source to be injected efficiently into an LGP, its thickness should be equal or larger than the source size. This is in particular relevant for LEDs, as most of the flux not entered into an LGP will be lost.
In some applications, there is a further requirement for an additional collimation of the source radiation for a more effective subsequent extraction in an LGP. In this case, as is illustrated in FIG. 1B, an LED 18 is coupled to a collimating section 20 at an edge aperture 22 having an edge aperture width of “Z”. Existing backlights for small aperture LCDs for cellular phones have a typical edge width “Z” of between 0.7-1.5 mm and suffer from coupling efficiency losses. Smaller values of Z are particularly beneficial for frontlights for reflective LCDs, since it allows one to achieve a smaller degradation of a displayed image, smaller parallax and less subjectively disturbing perceptual difference between the depth of frontlight and LCD planes. In other common applications, such as keyboards, dashboards and instrument lighting, FIDs frequently cannot be used at all since in addition to much smaller acceptable thickness they should be mechanically flexible to enable key operation and/or ultra compact packaging. As a result of this deficiency, multiple LEDs are used to directly illuminate localized display areas, resulting in high luminance non uniformity, low power efficiency, complicated mounting and elevated cost.
Regardless of the specific application, different types of LCDs have different requirements from an associated radiation management system, such as an LGP. Some of these are described in greater detail below.
Transmissive LCDs.
Portable and handheld communication and computing devices are a very attractive market for small and medium LCDs. The market is rapidly expanding, and these applications are a natural fit for LCDs because, among other advantages, they offer the thin profile necessary for creating small devices. However, there are a number of serious problems that must be addressed when using LCDs in these devices, most of which center around the backlight. For transmissive LCDs, the backlight saps much of the battery power because it must be very bright to shine through the strong absorption of the multilayered LCD. Color LCDs are particularly inefficient; efficiency losses due to the requisite polarizers, color filters, and “transparent” electrodes results in a final light transmission of only about 5%. Such low transmission makes the power consumption of color LCDs for most mobile applications pro-hibitively high. The battery would have to be enormous, or else the device would need new batteries far too frequently. Finally, there is the problem of poor view-ability in that the standard backlit transmissive LCD is nearly unreadable in sunlight. These issues can be addressed with varying results, in a number of ways including changing the method of light modulation and reducing the absorptive losses of the LCD components.
Reflective LCDs.
For reflective LCDs the backlight design also plays a pivotal role. Reflective LCDs can be made in two configurations: outer-surface reflective and-inner-surface reflective. Outer-surface reflective devices are constructed similarly to transmissive devices, with the backlight replaced by a reflective surface behind the rear polarizer. While simple, this configuration has the disadvantage of poor contrast and color at a wide viewing angle, because the incident and reflected beams pass through different color filters (leading to both greater absorption of the light and color mixing). There can also be parallax problems associated with having the reflector behind both glass substrates. Outer-surface reflection is widely used in watch and calculator displays (and has been for more than 20 years) and may be acceptable for handheld devices, but the quality is too poor for sophisticated displays such as notebook computers.
A superior construction (although more difficult to fabricate) is the inner-surface reflective device, which incorporates the reflective layer into the bottom electrode. Because the reflection occurs much closer to the color-filter layer, the viewing-angle problems are greatly diminished.
The reflective LCD is a tremendous boon for some mobile devices because it eliminates the need for a backlight. However, reflective LCDs cannot, of course, be used in darkness, which is a requirement for some devices, such as cellular telephones and PDAs. Hence, a hybrid device has evolved, called the transflective LCD. In transflective LCDs, both transmissive and reflective modes are employed; the device has a backlight, but it also makes use of ambient light through reflection. Seiko-Epson Corp. has developed a transflective configuration, which it calls semi-transparent, and which replaces the reflective layer in an outer-surface reflective device with a semitransparent plate having a given reflectance-transmittance ratio, for example 0.7 and 0.3 (if zero absorption is assumed). The backlight partially shines through, and ambient light is partially reflected. Seiko Epson has demonstrated a 6.5-inch thin-film diode transflective LCD. With half-VGA pixel format, the display acts like a transmissive product when the backlight is on and like a reflective product when the backlight is off. Seiko-Epson has also developed a reflective film for the display that has a 10:1 contrast ratio and a 512-color capability; power consumption is reported to be 0.12 watt. Most of the low cost reflective LCDs for portable devices incorporate a similar design using a transflector. However, such a transflector reduces both an effective reflectance of the LCD in a passive mode and an observable luminance in backlit mode in a proportion equal to its reflectance-transmittance ratio.
It would be highly advantageous to remove a transflector and to use a ˜100% reflectance reflector with a similar BRDF (bidirectional reflection distribution function) behind the backlight instead of a presently used white lambertian back reflector. Another apparent alternative is the use of a frontlighting scheme, whereby an illumination device is placed in front of the LCD. However, prior art frontlights employing microprismatic extractors have an inherently low luminous efficacy, large thickness, introduce image distortion and are expensive. In view of these disadvantages, their use has been limited.
Thus, there is a well recognized need for FIDs featuring drastically reduced thickness along with higher output luminous efficacy, greater degree of control over intensity distribution of the extracted radiation, better uniformity, better mechanical flexibility, ease of manufacturing and reduced cost. This need exists regardless of whether the FID is reflective, transmissive or transflective.